Satellite digital audio radio services (SDARS) have become increasingly popular, offering digital radio services covering large geographic areas, such as North America. XM Satellite Radio is one example of satellite digital audio radio services. These services typically receive uplinked programming, which, in turn, is rebroadcast directly to digital radios that subscribe to the service. Each subscriber to the service generally possesses a digital radio having a receiver and antenna for receiving the digital broadcast. Although many digital radios have been designed for use in vehicles, other digital radios are increasingly being designed for use in the home or office environment, and for personal portable or wearable usage, including in outdoor environments.
In SDARS systems, the digital satellite receivers are generally programmed to receive and decode digital satellite signals, which typically include many channels of digital audio. These signals are received directly from satellites, or from terrestrial repeaters that retransmit the digital satellite signals in order to provide improved coverage and availability. In addition to broadcasting encoded digital quality audio signals, the satellite service may also transmit data that may be used for various other applications. The broadcast signals may include advertising, information about warranty issues, information about the broadcast audio programs, and news, sports, and entertainment programming. Thus, the digital broadcasts may be employed for any of a number of satellite audio radio, satellite television, satellite Internet, and various other consumer services.
A signal received by a digital satellite receiver is generally filtered and amplified by circuitry in the receiver prior to being provided as an output signal from the receiver. One reason for filtering the received signal is to remove unwanted signal components associated with the received signal that can interfere with or distort the desired information contained in the received signal. Image frequencies are one example of unwanted signal components that can be removed by receiver filter circuitry. One technique used in receivers to suppress or remove Image frequency signals is to use an image reject filter. Typically, this is a band-pass filter with a good image reject ratio. If the output signal from the filter is an unbalanced signal, the output from the filter can be provided to a balun in order to convert the unbalanced signal from the filter into a balanced differential signal that can be processed by additional receiver circuitry, such as, for example, a mixer.
Typical desired characteristics of filters employed as discussed above may include low cost, small size to allow for efficient use of board or substrate real-estate, ease of integration with other circuit components, precise stop and pass-band frequencies, precise center frequencies, high image reject ratios, and low insertion loss. Typical desired characteristics of baluns employed as discussed above may include low cost, small size for efficient use of board or substrate real-estate, ease of integration with other circuit components, tight signal coupling between the balun input and output, output amplitude balance, phase balance and low insertion loss.
In traditional receiver designs, baluns and filters may be implemented in discrete, packaged devices that are mounted to the receiver printed circuit board or substrate. Discrete components implementing baluns, filters and combinations of baluns and filters are currently marketed by companies such as Toko, Inc. and Soshin Electric Co., Ltd. While the use of discrete baluns, filters, and combinations of baluns and filters can be effective in processing signals received by digital satellite receivers, such discrete components can consume scarce board or substrate real-estate, can be expensive, and can provide sub-optimal performance with respect to other important filter and balun characteristics noted above.
As an alternative to the discrete devices noted above, filters and baluns may be implemented as structures formed by embedded or implanted metal lines in multi-layer ceramic substrates such as low-temperature co-fired ceramic substrates (LTCC). Generally, these devices are formed using multiple layers in a multi-layer substrate. While these devices can be effective in processing signals received by digital satellite receivers, and can potentially reduce cost and board or substrate use relative to discrete components, process variation across layers and other factors can make it difficult to precisely align and form the elements needed to create filter and balun devices that closely meet design requirements.
When a bandpass filter is created in a multi-layer substrate, it is often created with parallel LC resonators formed from capacitors and inductors connected to ground planes through multiple vias. In order for the filter to perform as designed, it is typically desirable to have the inductance of the parallel resonators be as closely matched as possible. However, because each inductor is typically formed across multiple different layers in the substrate, process variation and differences in each layer make it difficult to match the inductances. Connecting the inductors to multiple ground planes through multiple vias can also make matching inductances difficult. This is due in part to the fact that the ground planes and vias may have different physical characteristics, causing them to contribute differently to the inductances of each LC resonator. Finally, attempts to create a bandpass filter with low insertion loss by altering the dielectric constant of layers in the vicinity of the LC resonator can make it difficult to match the inductances when the inductors are formed across multiple different layers.
When a balun is created in a multi-layer substrate, it is typically created by running an electromagnetically coupled input line and output line through different layers of the structure. Generally, the input line will be in one layer, the output line will be in a different layer, and the two lines are separated by a dielectric layer. It is generally desirable in a balun to have the input and output lines in separate layers overlapping each other so that the signals in each line are as tightly coupled as possible. However, process variations among the layers can make it difficult to have the path of the input line precisely match the path of the output line, resulting in unwanted coupling effects. In addition, attention must be paid to where the lines are run relative to other substrate structures in order to avoid unwanted coupling. The result is that the coupled lines might not be able to follow the most space-efficient path through the substrate. Finally, by only coupling the input line to one output line, the length of the 2 lines may be longer than necessary.
What is needed is a more effective balun filter combination embedded in a substrate, including a filter section with parallel LC resonator structures in which the inductance of the parallel structures can be closely matched in spite of process variation among different layers, changes to various layers to reduce insertion loss, and grounding of the inductors. What is also needed is a balun section in which input and output lines are more tightly coupled, less susceptible to coupling to other circuit structures, shorter, and more efficiently routed in order to reduce size.